The present invention relates in general to a duplex communication system and in particular to a duplex transmission system employing a two-wire link between a main station and a local station, the main station generating a clock signal whereby the coded data signal is transmitted during one half period of the main clock signal from the main station to the local station and during the other half period the coded data signal is transmitted from the local station to the main station.
A data transmission system of this type is known from the German Pat. No. 2,453,628. Such systems are employed for signal transmission over short distances and for a short duration of transmission. The two-wire link transmits in rapid direction exchange alternately from the main station to the local one and vice versa, information contained in a single signal step such as one bit, one dibit, or one tribit. The data transmission code is selected such that in the local station the rate or period of the main clock signal can be detected so that the local station is synchronized with the main station.
In the prior system according to the German Pat. No. 25 53 628 no active clock generator is present in the local station. As a consequence in the event of an interruption in the link, the local station is without clock signals.
Moreover, in the prior art system, the main station transmits to the local station a bipolar RZ (return to zero) signal and from the local station to the main station a unipolar RZ signal is transmitted. Most transmitted signals are scanned or sensed as to their timing by means of NRZ (non-return to zero) signals. In the RZ signals there is no possibility to achieve a transmission without direct current components.